Many years ago, I taught from a math text that gave "divisibility rules"
for the numbers one through ten. For example, "if the sum of the digits
of a number is divisible by three, the number is divisible by three." I
remember the rules for all the numbers except "7". I've looked in every
reference book I could think of, but have had no luck. Any chance you
could help me out here?

In a multiplication problem like "Three-quarters of _____ is ninety," how do you fill-in-
the-blank with the factor that satisfies the equality? Doctor Ian presents three different
methods: guess-and-improve; the equivalency between multiplication and division;
and multiplying through by a reciprocal.

I don't agree that subtraction and division are not commutative
operations. If we define a - b as a + (-b), then it commutes correctly
into (-b) + a. So why does everyone say subtraction is not commutative?